1. Field of the Invention
The present invention relates to a compressor for compressing refrigerants by driving a piston with a swash plate, and in particular, compressors utilized in air conditioning systems.
2. Description of the Related Art
A known piston-type compressor that is operated by a swash plate is described in Japanese Laid-Open Patent Publication Nos. 7-189897. Such a compressor includes a piston having an integrated piston head and engaging portion. The piston head is disposed in a cylinder bore formed in the housing of the compressor so that the piston head can reciprocate therein and the engaging portion engages with a pair of shoes attached to a pair of shoe seats. The pair of shoes includes front and rear shoes and a swash plate is disposed between the pair of shoes. The swash plate is supported by a rotatably supported drive shaft and rotates together with the drive shaft by means of a rotor mounted on the drive shaft. The angle of inclination of the swash plate changes with respect to a plane perpendicular to the axis of rotation of the swash plate in accordance with the cooling demand when the compressor is in operation. When the swash plate rotates, the piston head reciprocates in the cylinder bore via the interaction between the swash plate and the engaging portion of the piston, thereby compressing a refrigerant disposed within the cylinder bore. In this way, the rotational movement of the drive shaft is converted into a linear movement that drives the piston.
A piston rotation force is generated by the piston being drawn toward rotating direction of the swash plate due to friction resulting from a compression reaction force of the piston between the swash plate and shoe and friction between the shoe and piston. As the compression reaction force increases, the rotation force on the piston also increases. Therefore, a turn stop portion is provided on the engaging portion in the known compressor and contacts the inner wall surface of the housing in order to prevent the piston from rotating around its center axis.
As a result, the turn stop portion will forcefully contact the inner surface wall of the housing during operation of the compressor, thereby causing the turn stop portion and the housing to wear out prematurely. In particular, if a refrigerant is utilized that must be compressed under high pressure to provide adequate cooling, such as is necessary with carbon dioxide, the discharge pressure of the compressor is increased in comparison to when a low pressure refrigerant, such as a fluorocarbon, is used. Thus, wear on these contacting parts may be significantly increased if a known compressor design is used with a high compression refrigerant.
The piston also generates a compression reaction force, which applies a force to the sides of the piston head. This side force increases linearly with increases in the compression reaction force and results in friction between the cylinder head and the inner surface of the cylinder bore. If a high compression refrigerant is used in the known compressor, the discharge capacity of the compressor may be decreased because carbon dioxide has a higher compressibility and a higher density, for example, than fluorocarbon. In other words, the diameter of the piston head can be decreased if a high compression refrigerant, such as carbon dioxide, is used instead of a low compression refrigerant. However, if a high compression refrigerant is in fact used, the side force acting on the piston head will increase, because the compression reaction force of the piston also increases in comparison with the case in which a fluorocarbon is used. Therefore, if the piston diameter is decreased when a high compression refrigerant is used, premature wear on the piston head or the cylinder bore will be further increased by the combination of an increase in the side force and a decrease in the piston head area that receives the refrigerant pressure.